Maria Corina Machado, 56, an industrial engineer and former lawmaker, has led her rivals by some 40 points in polls.
The opposition and government this week signed a deal on some election guarantees, including the presence of international observers.
Some in the opposition have said they are skeptical Maduro will follow through on the deal.
The opposition, which says the disqualifications are unlawful, has been reticent to confirm what it would do if Machado wins the primary but is unable to compete in 2024.
Others have argued selecting a substitute candidate would be necessary, though whether the often-fractious opposition would accept Machado choosing a replacement remains to be seen.
Persons:
Leonardo Fernandez Viloria, Nicolas Maduro, Maria Corina Machado, Machado, Maduro, Jorge Rodriguez, Carlos Prosperi, Delsa, Mayela Armas, Vivian Sequera, Julia Symmes Cobb, Rod Nickel
Organizations:
El, REUTERS, Rights, Trump, Thomson
Locations:
El Valle, Venezuela, Caracas, Rights CARACAS, United States